The Ovonic EEPROM is a proprietary, high performance, non-volatile, thin-film electronic memory device. Its advantages include non-volatile storage of data, potential for high bit density and, consequently, low cost because of its small footprint and simple two-terminal device configuration, long reprogramming cycle life, low programming energies and high speed. The Ovonic EEPROM is capable of both analog and digital forms of information storage. Digital storage can be either binary (one bit per memory cell) or multi-state (multiple bits per cell).
The use of electrically writable and erasable phase-change materials (i.e., materials which can be electrically switched between generally amorphous and generally crystalline states or between different resistive states while in crystalline form) for electronic memory applications is well known in the art and is disclosed, for example, in commonly assigned U.S. Pat. No. 5,166,758, the disclosure of which is incorporated by reference herein. Other examples of electrical phase-change materials and memory elements are provided in commonly assigned U.S. Pat. Nos. 5,296,716, 5,414,271, 5,359,205, 5,341,328, 5,536,947, 5,534,712, 5,687,112, and 5,825,046 the disclosures of which are all incorporated by reference herein. Still another example of a phase-change memory element is provided in commonly assigned U.S. patent application Ser. No. 09/276,273, the disclosure of which is incorporated herein by reference.
Generally, the phase-change materials are capable of being switched between a first structural state where the material is generally amorphous and a second structural state where the material is generally crystalline local order. The term "amorphous", as used herein, refers to a condition which is relatively structurally less ordered or more disordered than a single crystal and has a detectable characteristic, such as high electrical resistivity. The term "crystalline", as used herein, refers to a condition which is relatively structurally more ordered than amorphous and has lower electrical resistivity than the amorphous state.
The phase-change materials may also be electrically switched between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. That is, switching of such materials is not required to take place between completely amorphous and completely crystalline states but rather the material can be switched in incremental steps reflecting (1) changes of local order, or (2) changes in volume of two or more materials having different local order so as to provide a "gray scale" represented by a multiplicity of conditions of local order spanning the spectrum between the completely amorphous and the completely crystalline states.
The phase-change material exhibits different electrical characteristics depending upon its. state. For instance, in its crystalline, more ordered state the material exhibits a lower electrical resistivity than in its amorphous, less ordered state. The volume of phase-change material is capable of being switched between a more ordered, low resistance state and a less ordered, high resistance state. The low resistance state has a resistance value which is less than the resistance value of the high resistance state. Furthermore, the resistance values of the high and low resistance states are detectably distinct.
The volume of phase-change memory material is capable of being transformed from its high resistance state to its low resistance state when a relatively long pulse of energy, referred to as a "set pulse", is applied to the material. While not wishing to be bound by theory, it is believed that application of a set pulse to the volume of memory material changes the local order of at least a portion of the volume of material. Specifically, the applied energy causes at least a portion of the volume of memory material to change from its less-ordered "amorphous" condition to a more-ordered "crystalline" condition by causing some amount of crystallization (i.e., nucleation and/or growth).
Application of additional consecutive set pulses promotes further crystallization of the memory material. As consecutive set pulses are applied, new nucleation sites are formed and existing crystallites increase in size. Increased crystallite size creates stress at the boundaries between the memory material and other layers of the memory element (for example, between the memory material and the contact layer material), thereby increasing the tendency for delamination of the memory element.
There is thus a need for a method of programming the memory element which ensures that consecutive set pulses are not applied to the volume of memory material.